OK, I'll nominate one of the most neglected and obscure works by Edgar Allen Poe--The Narrative of Arthur Gordon Pym of Nantucket. It is the only novel length work by Poe and while Poe based it on several real-life events and apparently intended at the beginning to make it realistic, it very soon became very strange. The novel begins as an adventure story involving a stowaway, Arthur Gordon Pym, and continues with his ever more weird adventures including shipwreck, mutiny, and cannibalism. Poe published it in 1838 and it received very mixed reviews.

Goodreads says this about it:
"Some critics responded negatively to the work for being too gruesome & for cribbing heavily from other works, while others praised its exciting adventures. Poe himself later called it "a very silly book". Nevertheless, The Narrative became an influential work, notably for Herman Melville & Jules Verne."

Despite Poe's dismissive remark, it remains an interesting oddity.

To be honest, I've never read it through entirely and perhaps this might provide an opportunity to do so.

It's available as a free download from Project Gutenberg. It is the main text in Works of Edgar Allen Poe--Volume 3.